Over at the Ms. blog, Rachel Kassenbrock posted an adorable video of two Capuchin monkeys given unequal rewards for the same task. The monkey who gets the lesser reward displays her displeasure, in no uncertain terms.

The research, described by primatologist Frans de Waal, has been reproduced in chimps, dogs, and birds. It reveals an inherent sense of fairness, suggesting that social justice may not be a silly fantasy, but quite natural indeed.